CIE A Level Chemistry

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A LevelChemistryCIETopic Questions5. Physical Chemistry (A Level Only)5.5 Equilibria (A Level Only)

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First teaching 2020

Last exams 2024

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5.5 Equilibria (A Level Only)

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1a1 mark

A saturated solution of silver(I) chloride has a solubility of 1.46 x 10-3 mol dm-3.

Write the expression for the solubility product, Ksp, for silver(I) chloride.

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1b
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Using the information and your answer given in part (a), calculate the solubility product, Ksp, and its units for silver chloride. Show your working.

Ksp = .........................

units = ........................

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1c
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A saturated solution of iron(II) hydroxide has a solubility of 5.82 x 10-6 mol dm-3.

i)
Write the expression for the solubility product, Ksp, for iron(II) hydroxide.
 
[1]
 
ii)
Calculate the solubility product and units for iron(II) hydroxide. Show your working
 
Ksp = ...................
 
units = ..................

[3]

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2a
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The general equilibrium expression for the partition coefficient is

Partition coefficient = begin mathsize 14px style fraction numerator open square brackets equilibrium space concentraton space in space mol space dm to the power of negative 3 end exponent space in space organic space layer close square brackets over denominator open square brackets equilibrium space concentration space in space mol space dm to the power of negative 3 end exponent space aqueous space layer close square brackets end fraction end style

100 cm3 of a 0.5 mol dm-3 solution of aqueous ethylamine, CH3CH2NH2, was shaken with 100 cm3 of an organic solvent at 25 °C and left in a separating funnel to allow an equilibrium to established. The equilibrium equation is:

CH3CH2NH2 (aq) rightwards harpoon over leftwards harpoon CH3CH2NH2 (organic solvent)

Only 50 cm3 of the aqueous layer was run off and titrated against 0.50 mol dm-3 dilute hydrochloric acid. 15.0 cm3 of hydrochloric acid was required to reach the end point.

CH3CH2NH2 (aq) + HCl (aq) → CH3CH2NH3+ (aq) + Cl– (aq) 

i)
Calculate the amount, in moles, of ethylamine that has reacted with hydrochloric acid.



moles of ethylamine that reacted = ....................... mol
[1]
 
ii)
Calculate the amount, in moles, of ethylamine in the aqueous layer.



moles of ethylamine in aqueous layer = ....................... mol
[1]
 
iii)
Calculate the amount, in moles, of ethylamine in the organic layer.



moles of ethylamine in organic layer = ....................... mol
[1]
 
iv)
Calculate the concentration, in mol dm-3, of ethylamine in the aqueous and organic layer.





Concentration in aqueous layer = ................... mol dm-3
 
Concentration in organic layer = ..................... mol dm-3
[1]
 
v)
Calculate the partition coefficient, Kpc, of ethylamine between water and the organic solvent.



Kpc = ..................
[1]

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2b2 marks

Use your answer to part (a) to determine whether ethylamine is more soluble in the aqueous or organic solvent. Explain your answer.

Solvent .................................................

Explanation ..............................................................................................

                   ................................................................................................

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2c
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A solution containing 1.00 g of compound A in 100 cm3 of water was shaken with 10 cm3 of ether. 0.60 g of A was transferred to the ether layer.

Calculate the partition coefficient of compound A between ether and water. Show your working.

 

 

 

 

Kpc = ........................

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3a1 mark

The Ksp value for silver bromide, AgBr (s) is 3.3 x 10-13 at 298 K.

Write the equilibrium equation for the changes that happen in a saturated solution of silver bromide in the presence of some solid.

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3b2 marks

A solution of sodium bromide was added to a saturated solution of silver bromide solution. How would the concentration of dissolved silver ions change. Explain your answer.

Change in concentration of silver ions ..................

Explanation ................................................................................

      ................................................................................

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3c
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Calculate the concentration, in mol dm-3, of silver ions at 298 K in a saturated solution of silver bromide. Show your working.





[Ag+ (aq)] = ....................... mol dm-3

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4a3 marks

Acidic buffer solutions are made of a weak acid and its salt. pH calculations of buffer solutions can be conducted using the same equation used for weak acids:

Ka = fraction numerator left square bracket straight H to the power of plus right square bracket left square bracket straight A to the power of minus right square bracket over denominator left square bracket HA right square bracket end fraction

State the assumptions made when using this equation for buffer solutions.

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4b1 mark

Suggest a salt that could be used to form a buffer solution with propanoic acid, CH3CH2COOH.

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4c4 marks

A buffer solution contains a 0.15 mol dm-3 concentration of propanoic acid and a propanoate salt with a concentration of [A–] = 0.30 mol dm-3.

At 298 K the value of Ka for propanoic acid is 1.35 × 10–5 mol dm–3.  

i)
Calculate the concentration of [H+] in this buffer solution at 298 K. Give your answer to 2 decimal places. Show your working.
 
[H+] = ...................... mol dm-3
[3]
 
ii)
Calculate the pH of the buffer solution to 2 decimal places.
 
pH = .....................
[1]

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4d2 marks

Buffer solutions can be tailored to a specific pH by 'coarse-tuning' by changing the acid used, and 'fine-tuning' by altering the ratio of [HA] : [A–].

Predict and explain the effect on the pH of the buffer solution by increasing the concentration of the salt only.

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5a2 marks

Two acids used in the same laboratory, which both have the same concentration, 0.1 mol dm-3, can have significantly different pH values. Explain why.

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5b
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Calculate the pH of a solution of 0.1 mol dm-3 HCl.

 Show your working


pH = .........................

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5c
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A sample of 0.1 mol dm-3 methanoic acid has a Ka value of 1.78 x 10-4 mol dm-3 at 298 K.

i)
Write an expression for the acid dissociation constant, Ka, for methanoic acid.
 
         Ka = 
[1]
 
ii)
Calculate the pH of the 0.1 mol dm-3 methanoic acid. Give your answer to two decimal places and show your working.
  
 
pH = ....................
[3]

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1a2 marks

3-dodecylbenzenesulfonic acid can be prepared from benzenesulfonic acid.

benzenesulfonic-acid

Suggest the reagents and conditions and name the mechanism for this reaction.

reagents and conditions ..................................................................................................

mechanism ............................................................................................

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1b2 marks

When concentrated sulfuric acid is added to water, dissociation takes place in two stages.

   stage 1 H2SO4 rightwards harpoon over leftwards harpoon H+ + HSO4–

   stage 2 HSO4– rightwards harpoon over leftwards harpoon H+ + SO42–     Ka2 = 1.0 × 10–2 mol dm–3

Ka2 is the acid dissociation constant for stage 2.

i)
Write the expression for the acid dissociation constant Ka2.
 
Ka2 =
 
[1]
 
ii)
H2SO4 is considered a strong acid whereas HSO4– is considered a weak acid.
 
Suggest how the magnitude of the acid dissociation constant for stage 1 compares to Ka2.
 
[1]

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1c
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Benzoic acid, C6H5CO2H, is a weak acid.

The value of the acid dissociation constant, Ka, of benzoic acid is 6.3 x 10-5 mol dm-3 at 298K.

Calculate the pH of the 0.50 g dissolved in 250 cm3 of water.

Give your answer to two decimal places and show your working.





pH = .....................

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1d
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Calculate the pH of the solution formed when 0.060 g of sodium hydroxide is added to 500.00 cm3 of 0.005 mol dm-3 of benzoic acid. Give your answer to two decimal places. Show your working.




pH = ................. 

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2a5 marks

One atom of each of the four elements H, C, N and O can bond together in different ways.

Two examples are molecules of cyanic acid, HOCN, and isocyanic acid, HNCO. The atoms are bonded in the order they are written.

i)
Draw ‘dot-and-cross’ diagrams of these two acids, showing outer shell electrons only.
 







HOCN, cyanic acid







HNCO, isocyanic acid
 
[3]
 
ii)
Suggest the values of the bond angles HNC and NCO in isocyanic acid.
 
HNC ..............................
 
NCO ..............................
 
[1]
 
iii)
Suggest which acid, cyanic or isocyanic, will have the shorter C–N bond length. Explain your answer.
 
[1]

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2b3 marks

Isocyanic acid is a weak acid.

HNCO rightwards harpoon over leftwards harpoon H+ + NCO–    Ka = 1.2 × 10–4 mol dm–3

i)
Calculate the pH of a 0.10 mol dm–3 solution of isocyanic acid and show your working.
 
 
 
pH = ..............................
[2]
 

Sodium cyanate, NaNCO, is used in the production of isocyanic acid. Sodium cyanate is prepared commercially by reacting urea, (NH2)2CO, with sodium carbonate. Other products in this reaction include ammonia and steam.

ii)
Write an equation for the production of NaNCO by this method.
 
[1]

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2c
Sme Calculator7 marks

Barium hydroxide, Ba(OH)2, is completely ionised in aqueous solutions. During the addition of 30.0 cm3 of 0.100 mol dm–3 Ba(OH)2 to 20.0 cm3 of 0.100 mol dm–3 isocyanic acid, the pH was measured.

i)
Calculate the [OH–] at the end of the addition.
 
[OH–] = .............................. mol dm–3 
[2]
 
ii)
Use your value in (i) to calculate [H+] and the pH of the solution at the end of the addition.
 
final [H+] = .............................. mol dm–3
 
final pH = ..............................
[2]
 
iii)
On the following axes in Fig. 2.1, sketch how the pH changes during the addition of a total of 30.0 cm3 of 0.100 mol dm–3 Ba(OH)2 to 20.0 cm3 of 0.100 mol dm–3 isocyanic acid.
 ph-axes
 
Fig. 2.1
 
[3]

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3a
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This question is about buffer solutions

Calculate the pH of a buffer solution made by dissolving 0.275 g of benzoic acid (Ka  = 6.3 x 10-5 mol dm-3) and 0.525 g of sodium benzoate in 250 cm3 of water. Give your answer to 2 decimal places and show your working.





pH = .................

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3b
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Calculate the pH of a buffer solution made by mixing together 130 cm3 of 0.2 mol dm-3 ethanoic acid (Ka = 1.74 x 10-5 mol dm-3) and 85 cm3 of 0.45 mol dm-3 sodium ethanoate. Give your answer to 2 decimal places and show your working.

 






pH = .....................

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3c
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Calculate the new pH of the buffer solution given in part (b) if 1.00 cm3 of 1.00 mol dm-3 sodium hydroxide is added. Give your answer to two decimal places and show your working. 

Comment on the pH value after the addition of sodium hydroxide.

pH = ...................

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3d2 marks

Write two equations for the separate addition of a small volume of base and acid to the buffer solution given in part (b).

 

equation 1 ....................................................................................

 

equation 2 .....................................................................................

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1a
Sme Calculator6 marks
i)

State what is meant by partition coefficient.

[2]

Ammonia is soluble in both water and organic solvents.

An aqueous solution of ammonia is shaken with the immiscible organic solvent trichloromethane. The mixture is left to reach equilibrium.

Samples are taken from each layer and titrated with dilute hydrochloric acid.

  • A 25.0 cm3 sample from the trichloromethane layer requires 13.0 cm3 of 0.100 mol dm–3 HCl to reach the end-point.
  • A 10.0 cm3 sample from the aqueous layer requires 12.5 cm3 of 0.100 mol dm–3 HCl to reach the end-point.
ii)

Calculate the partition coefficient, Kpc, of ammonia between trichloromethane and water.

Show your working.

Kpc = ...........................................................

[2]

iii)

Butylamine, C4H9NH2, is also soluble in both water and organic solvents.

Suggest how the numerical value of Kpc of butylamine between trichloromethane and water would compare to the value of Kpc calculated in (a)(ii). Explain your answer.

[2]

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1b1 mark

Butanamide, C3H7CONH2, is much less basic than butylamine. Explain why.

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2a3 marks

The feasibility of a chemical reaction depends on the standard Gibbs free energy change, ΔGɵ.
This is dependent on the standard enthalpy and entropy changes, and the temperature.

State and explain whether the following processes will lead to an increase or decrease in entropy.

i)
the reaction of magnesium with hydrochloric acid

entropy change ....................................

explanation ..............................................

[1]

ii)
the dissolving of solid potassium chloride in water

entropy change ....................................

explanation ...........................................

[1]

iii)
the condensing of water from steam

entropy change ....................................

explanation .............................................

[1]

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2b
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Magnesium carbonate can be decomposed on heating.

MgCO3 (s) → MgO (s) + CO2 (g)      ΔHɵ = +117 kJ mol–1


Standard entropies are shown in Table 2.1.

Table 2.1

substance MgCO3 (s) MgO (s) CO2 (g)
Sɵ / J K–1 mol–1 +65.7 +26.9 +214

i)
Calculate ΔGɵ for this reaction at 298 K.
 
Show your working. 
 
ΔGɵ = ............................................. kJ mol–1 
 
[3]

ii)
Explain why this reaction is feasible only at high temperatures.
 
[1]

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2c1 mark

Table 2.2 lists values of solubility products, Ksp, of some Group 2 carbonates.

Table 2.2

  solubility product in water at 298 K, Ksp / mol2 dm–6
MgCO3 1.0 × 10–5
CaCO3 5.0 × 10–9
SrCO3 1.1 × 10–10

Deduce the trend in the solubility of the Group 2 carbonates down the group. Justify your answer using the data given.

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2d3 marks
i)
Write an equation to show the equilibrium for the solubility product of MgCO3. Include state symbols.
 
..................................... ⇌ ..................................................
 
[1]
 
ii)
With reference to your equation in (d)(i), suggest what is observed when a few cm3 of concentrated Na2CO3 (aq) are added to a saturated solution of MgCO3. Explain your answer.
 
[2]

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2e
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Use the data in Table 2.2 to calculate the solubility of MgCO3 in water at 298 K, in g dm–3.

 

Show your working.

 
solubility of MgCO3 = ................................................ g dm–3

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2f3 marks

Describe and explain the variation in the thermal stabilities of the carbonates of the Group 2 elements.

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3a
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The pH of a 0.150 mol dm–3 solution of piperidine is 11.9.

20.0 cm3 of 0.200 mol dm–3 HCl was slowly added to a 20.0 cm3 sample of 0.150 mol dm–3 piperidine. The pH was measured throughout the addition. One mole of hydrochloric acid reacts one mole of piperidine.

i)
Calculate the number of moles of HCl remaining at the end of the addition.




moles of HCl = .....................
[2]
ii)
Hence calculate the [H+] and the pH at the end of the addition.




pH = .....................
[1]

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3b
Sme Calculator3 marks

On the following axes shown in Fig. 3.1, sketch how the pH will change during the addition of a total of 20.0 cm3 of 0.200 mol dm–3 HCl.

5-5-3b-m-ph-curve---addition-of-hcl-a
Fig. 3.1

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3c5 marks

The student repeated the titration using two different chemicals, 25.0 cm3 of 0.100 mol dm-3 nitric acid, HNO3 (aq), and 0.100 mol dm-3 ammonia, NH3 (aq).
 

i)
State the equation for this reaction. 
[1]

ii)
Explain why the salt produced in this reaction is acidic.
[4]

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3d2 marks

State the equation for the reaction between nitric acid and water and identify the conjugate acid formed in the reaction. 

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4a1 mark

This question is about buffers.

Give the meaning of the term buffer solution.

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4b2 marks

Buffers can be acidic but they can also be basic. This is determined by the reactants which are combined to create the buffer

i)
Describe how an acidic buffer is made.

ii)
State the key feature which makes a buffer acidic.

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4c4 marks

Ethanoic acid is a weak acid. Hydrogen carbonate ions act as a weak acid if they are in an aqueous solution.

i)

Write equations for each of these weak acids at equilibrium. 

[2]

ii)

A solution was made up containing sodium hydrogen carbonate and sodium carbonate. Explain how this solution would act as a buffer if a small amount of acid was added to it.

[2]

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4d2 marks

Explain how the solution of ethanoic acid works as a buffer when small amounts of alkali are added.

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4e
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A buffer solution contains 0.10 mol dm-3 concentration of methanoic acid and a 0.20 mol dm-3 concentration of sodium methanoate. The values of Ka for methanoic acid is 1.80 × 10–5 mol dm–3. 


Calculate the pH of this buffer solution at 298 K. Give your answer to 2 decimal places and show your working. 



pH = .......................

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5a
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This question is about solubility products

A 1.70 x 10-3 mol dm-3 solution of calcium nitrate is mixed with an equal volume of 1.50 x 10-3 mol dm-3 potassium sulfate.

Predict whether calcium sulfate (Ksp = 2.00 x 10-5) will precipitate. Show your working.

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5b
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Water with a manganese ion, Mn2+ (aq), concentration above 1.8 x 10-6 mol dm-3 will cause clothes to stain when being washed. The manganese can precipitate as Mn(OH)2 (s) which has a Ksp of 4.5 x 10-14. This reduces this concentration of Mn2+ (aq) by the addition of OH– ions. 

i)
State the expression for Ksp 


[1]
ii)
Calculate the concentration of OH– ions. Show your working. 





[OH–] = ...................... mol dm-3
[2]
iii)
Calculate the minimum pH at 298 K required to prevent clothes from being stained.
(Kw = 1.00 × 10–14 mol2  dm–6 (at 298 K (25 °C))


pH = ...................
[2]

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5c
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Consider the following equilibrium containing a saturated solution of AgBr (s)

AgBr (s) rightwards harpoon over leftwards harpoon Ag+ (aq) + Br– (aq)

Explain what will happen to the solubility of AgBr (s) if the following substances are added to the solution.

  • AgNO3
    ..................................................................................................................

  • NaBr
    ..................................................................................................................

  • Pb(NO3)2
    .................................................................................................................. 

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5d3 marks

Explain why more Mg(OH)2 dissolves when hydrochloric acid is added to a saturated solution of Mg(OH)2.

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